1. Field of the Invention
The present invention relates to a signal processing device for creating a pulse modulated signal (for example, a PWM signal or a PDM signal) from a digital audio signal, and a class D amplifier for creating a pulse modulated signal from a digital audio signal and for amplifying the pulse modulated signal.
2. Description of Related Art
A prior art class D amplifier is provided with a driver circuit for amplifying an incoming digital audio signal by switching on two or more transistors (see patent reference 1, for example). In the driver circuit, a large amount of passing through current flows between a power supply and a ground according to the period of the digital audio signal, and radiation noise occurs due to the passing through current. When the digital audio signal has a large number of channels, transistors for different channels may be switched on at the same timing and, in this case, plural corresponding passing through currents are merged and therefore radiation noise increases.
[Patent reference 1] Japanese patent application publication No. 2001-223537 (see paragraph [0014] and FIG. 1)
A problem with a prior art class D amplifier constructed as mentioned above is that since transistors for different channels are switched on at the same timing more frequently as the number of channels of a digital audio signal inputted thereto increases, plural corresponding passing through currents are merged and therefore radiation noise increases.